Well JB weld is junk when trying to use it around gas or diesel. Like Steve said the best thing would be to have it repaired. Several of the radiator shops around here will also repair fuel tanks.
If you are going to try some other type of sealer get soem that is made for gas/diesel. I have used a product called "Seal All". It is mad efor use around fuels. Clean up any oil around wehre you need to repair your tank. Then apply a thin layer of the Seal ALL. Then press some screen wire into it and cover than with more Seal ALL. I have done this several times with good results. Seal All is sold at various places. I even saw it at the Dollar store for $2.49 for a little tube. They have mnany sizes of tubes. So look around for the size you need.